I had this image buried with a series of other ducks and didn’t look all that close. However, after I went through the files I noticed it, A Green-winged Teal.
The South Carolina Lowcountry is a winter stop for these birds. They breed in northern most US and Canada.
They are smaller and tend to blend in with flocks of other ducks.
The female looks like a Gadwall to me so that’s probably why I missed this in the first pass.